Galápagos Giant Tortoises (Environmental factors of Evolution)
They are also the world's largest tortoises, with some specimens exceeding five feet in length and reaching more than 500 pounds (226kg). Giant tortoises were once so abundant on the Galápagos archipelago off Ecuador that the Spanish sailors who explored the region in 1535 named the string of islands for them. (The Spanish word for tortoise is galápago.) Although the islands were once thought to be home to at least 250,000 tortoises, only about 15,000 remain in the wild today. Many of the tortoise's subspecies are listed by the International Union for Conservation of Nature as endangered or critically endangered.
All species of Galápagos tortoises evolved from common ancestors that arrived from mainland South America by overwater dispersal. Genetic studies have shown that the Chaco tortoise of Argentina and Paraguay is their closest living relative. Mitochondrial DNA analysis indicates that the oldest existing islands (Española and San Cristóbal) were colonised first, and that these populations seeded the younger islands via dispersal in a "stepping stone" fashion via local currents. Restricted gene flow between isolated islands then resulted in the independent evolution of the populations into the divergent forms observed in the modern species. The evolutionary relationships between the species thus echo the volcanic history of the islands.
The Tortoise Body & Shell Variations (Grazers and Browsers)
Galapagos Islands tortoises evolved with predominately two different type shells, "domed," left, and "saddleback." Dr. Ylenia Chiari's research suggests the energy exerted by the tortoises as they righted themselves may have led to the differences. Photos courtesy of Dr. Ylenia Chiari.Galápagos tortoises possess two main shell forms that correlate with the biogeographic history of the species group. They exhibit a spectrum of carapace morphology ranging from "saddleback" (denoting upward arching of the front edge of the shell resembling a saddle) to "domed" (denoting a rounded convex surface resembling a dome). When a saddleback tortoise withdraws its head and forelimbs into its shell, a large unprotected gap remains over the neck, evidence of the lack of predation during the evolution of this structure. Larger islands with humid highlands over 800 m (2,600 ft) in elevation, such as Santa Cruz, have abundant vegetation near the ground. Tortoises native to these environments tend to have domed shells and are larger, with shorter necks and limbs. Saddleback tortoises originate from small islands less than 500 m (1,600 ft) in elevation with dry habitats (e.g. Española and Pinzón) that are more limited in food and other resources.
Two lineages of Galápagos tortoises
possess the Island of Santa Cruz and when observed it is concluded that despite
the shared similarities of growth patterns and morphological changes observed
during growth, the two lineages and two sexes can be distinguished on the basis
of distinct carapace features. Lineages differ by the shape of the vertebral
and pleural scutes. Females have a more elongated and wider carapace shape than
males. Carapace shape changes with growth, with vertebral scutes becoming
narrower and pleural scutes becoming larger during late ontogeny.
Saddleback
tortoises require a higher energy input than domed species to successfully
self-right. Saddleback tortoises self-right by pushing their head on the ground
and then using their feet to flip over, Domed tortoises rely on moving their
feet and head to gain sufficient momentum to self-right. These traits could
have evolved to facilitate self-righting.
The Greatest Show on Earth: The Evidence for Evolution Book by Richard Dawkins
This is a book that I thoroughly enjoyed and learned from allot.
This is also a great book if you are looking to investigate the evidence for evolution in all the natural science fields (as the title suggests).
Here is a small part of the books content discussing the Galapagos giant tortoises:
All the
Galapagos giant tortoises are similar to a particular mainland species of land
tortoise, Geochelone chilensis, which is smaller than any of them. At some
point during the few million years that the islands have existed, one or a few
of these mainland tortoises inadvertently fell in the sea and floated across.
How could they have survived the long and doubtless arduous crossing? Surely
most of them didn’t. But it would have only taken one female to do the trick.
And tortoises are astonishingly well-equipped to survive the crossing. The
early whalers took thousands of giant tortoises from the Galapagos islands away
in their ships for food. To keep the meat fresh, the tortoises were not killed
until needed, but they were not fed or watered while waiting to be butchered.
They were simply turned on their backs, sometimes stacked several deep, so they
couldn’t walk away. I tell the story not in order to horrify (although I have
to say that such barbaric cruelty does horrify me), but to make a point. Tortoises
can survive for weeks without food or fresh water, easily long enough to float
in the Humboldt Current from South America to the Galapagos archipelago. And
tortoises do float. Having reached and multiplied upon their first Galapagos
island, the tortoises would with comparative ease – again accidentally – have
island-hopped the much shorter distances to the rest of the archipelago by the
same means. And they did what many animals do when they arrive on an island:
they evolved to become larger. This is the long-noticed phenomenon of island
gigantism (confusingly, there is an equally well-known phenomenon of island
dwarfism).
If the
tortoises had followed the pattern of Darwin’s famous finches, they would have
evolved a different species on each of the islands. Then, after subsequent
accidental driftings from island to island, they would have been unable to
interbreed (that’s the definition, remember, of a separate species) and would
have been free to evolve a different way of life uncontaminated by genetic swamping.
You could say that the different species’ incompatible mating habits and
preferences constitute a kind of genetic substitute for the geographic
isolation of separate islands. Though they overlap geographically, they are now
isolated on separate ‘islands’ of mating exclusivity. So they can diverge yet
further. The Large, the Medium and the Small Ground Finch originally diverged
on different islands; the three species now coexist on most of the Galapagos
islands, never interbreeding and each specializing in a different kind of seed
diet.
The
tortoises did something similar, evolving distinctive shell shapes on the
different islands. The species on the larger islands have high domes. Those on
the smaller islands have saddle-shaped shells with a high-lipped window at the
front for the head. The reason for this seems to be that the large islands are
wet enough to grow grass, and the tortoises there are grazers. The smaller
islands are mostly too dry for grass, and the tortoises resort to browsing on
cactuses. The high-lipped saddle shell allows the neck to reach up to the
cactuses which, for their part, grow higher in an evolutionary arms race
against the browsing tortoises.
The
tortoise story adds to the finch model the further complication that, for tortoises,
volcanoes are islands within islands. Volcanoes provide high, cool, damp, green
oases, surrounded at low altitude by dry lava fields which, for a grazing giant
tortoise, constitute hostile deserts. Each of the smaller islands has a single
large volcano and its own single species (or sub-species) of giant tortoise
(except those few islands that have none at all). The big island of Isabela
(‘Albemarle’ to Darwin) consists of a string of five major volcanoes, and each
volcano has its own species (or sub-species) of tortoise. Truly, Isabela is an
archipelago within an archipelago: a system of islands within an island. And
the principle of islands in the literal geographical sense, setting the stage
for the evolution of islands in the metaphorical genetic sense of species, has
never been more elegantly demonstrated than here in the archipelago of Darwin’s
blest youth.
https://www.amazon.com/Greatest-Show-Earth-Evidence-Evolution/dp/1416594795
https://www.goodreads.com/book/show/6117055-the-greatest-show-on-earth
References, bibliography & further reading:
https://www.southalabama.edu/departments/publicrelations/pressreleases/120417tortoise.html
https://www.wired.com/story/resurrecting-a-long-lost-galapagos-giant-tortoise/
https://www.nationalgeographic.com/animals/reptiles/facts/galapagos-tortoise
https://en.wikipedia.org/wiki/Gal%C3%A1pagos_tortoise
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